The present invention relates to a door-like closure in general, and more particularly to a safety closure for closing traffic passages of high-risk or high-security installations, such as nuclear power plants, banks, prisons and similar installations.
It is already known to close such passages by door-like closures including a multi-layer protective plate which has a limited endurance for mechanical, thermal, or chemical damaging influences. In many applications, closures of these properties perform the function to satisfaction. However, nuclear power plants, banks, prisons and similar installations are structures which, because of the functions entrusted to them, are exposed to very high security risks. Therefore, it is desired to protect at least some of the passages of these installations by resorting to the use of corresponding closures which are constructed in such a manner that intruders, such as saboteurs, are prevented from entering those structures, which need be highly protected, even when they resort to such measures of mechanical, thermal or chemical nature as to at least partially damage or destroy the above-mentioned closure, at least to the extent necessary for reaching beyond the closure through the destroyed region in order to manipulate certain components or objects located beyond the closure, such as the locking mechanism of the closure.
The requirements which are to be satisfied under these circumstances by the closure of this type are expressed, among others, in a so-called endurance coefficient which is expressed in the terms of the time period for which the closure offers an effective resistance to the destruction thereof when tools, such as drills or cutting torches or the like are used which are still portable.
In this context, there has been already proposed, for use in a door-like closure of the type here under consideration, a multilayer protective plate which has such a composition as to be capable of withstanding any attempts at destruction thereof for an endurance period of an hour at most. This protective plate which is fully suited for withstanding the action of such means for destroying the same which can be usually transported by the intruders to the location of the closure, offers only a small resistance to forces resulting from the explosion of relatively small explosive charges in the order of magnitude corresponding to less than 1 kilogram of TNT (trinitrotoluol). This means that saboteurs in possession of transportable explosive charges in the order of magnitude of merely 3 kilogram TNT can substantially damage such a protective plate with a relative ease and rapidly. With these relatively small explosive charges, especially when the explosive is a plastic explosive, holes can be produced in the closure, when the explosive is applied in a concentrated manner, which have such dimensions as to enable the person using the charge to reach beyond the closure and manipulate the objects or components located behind the closure. This, of course, is very disadvantageous.